In ultrasonic converters have ultrasonic sensors and proximity switches conventional a relatively narrow acoustic lobe characteristics, and at the same time have comparatively low secondary lobe amplitudes. The quarter-wave transformer element serves to emit and receive ultrasonic waves in a gaseous propagation medium, preferably for use in ultrasonic proximity switches.
German Patent No. 39 11 047 C2 has already disclosed an ultrasonic transducer having a piezoceramic element and a transformer element, in which the surface of the transformer element facing away from the piezoceramic element has a surface area which is slightly different as compared with the base area of the piezoceramic element and with the area of the transformer element which faces the latter. In this context, the lateral line of the transformer element is divergent or convergent, with a straight, concave, or convex profile. The acoustic lobe characteristic achievable therewith, having considerable secondary lobe amplitudes of approximately -19 dB with respect to the principal lobe amplitude, is insufficient for use of the ultrasonic transducer, for example, in confined environmental conditions and in the presence of lateral interfering objects.
European Patent No. 0 655 156 describes an ultrasonic transducer having a transformer element with a cylindrical enveloping surface, which has an indentation on the enveloping surface and/or on the surface facing the piezoceramic element. This indentation allows highly directional acoustic lobes with low secondary lobe amplitudes of less than -30 dB.